1. Field of the Invention
The invention relates to an arrangement for reading information from a record carrier, the arrangement comprising:
a read head (Rmr1),
amplifier means (74) for amplifying the signal read by the read head,
equalizer means (76) for equalizing the signal read by the read head,
bit detector means (80) for detecting bits in the signal read by the read head and to a hard disk drive provided with the arrangement.
2. Description of the Related Art
Such arrangement is known from U.S. Pat. No. 5,559,646 and is used for reading information from a hard disk. It should however be noted that the invention is not restricted to an application in the magnetic field. The invention is equally well applicable in the optical field. Therefore, in such application in the optical field, a read head understood to be a photo diode, as is used in optical recording, and where the signal shift, to be discussed hereafter, could have been caused by a fingerprint on the optical disk, as an example.
Amplifiers for magneto-resistive (MR) heads sense very small variations in the MR resistance (typically 1%), and amplify these signals. MR elements are biased by either a voltage or a current. Variations in the resistance of the MR heads, due to a magnetic field, result in small signal voltages and/or currents which can be detected by the sense amplifiers. The biasing of the MR heads causes a DC voltage across the head. MR sense amplifiers must be AC coupled to the heads. To avoid filtering at the low end of the data frequency spectrum, which results in data distortion, the AC coupling has a long time constant, typically at 1 xcexcs. Large transients, such as those caused during thermal asperities, pass the AC coupling and can cause the amplifier to overload, resulting in loss of data during long periods.
The invention aims at providing an improved arrangement for reading information from the record carrier. In accordance with the invention, the arrangement is characterized in that the arrangement further comprises
DC correction means (78) for subtracting a correction value from an input signal in response to an error signal so as to obtain a DC correction output signal, the bit detection means (80) being adapted to detect a sequence of bits based in the DC corrected output signal,
error signal generating means (82,83,85) for generating the error signal in response to the corrected output signal and the sequence of bits.
The invention is based on the following recognition.
The MR heads fly above the disk at very low heights (typically 10 nm). When a collision with a particle on the disk takes place, kinetic energy is transformed to heat. The resistance of the MR elements increases (maximally some 10-20%). As the bias current remains constant, the baseline voltage (that is: the low frequency component of the output signal) increases rapidly. Further, the signal amplitude of the wanted signal changes by not more than the same 10-20%. After the thermal asperity, the MR element cools down slowly. These large unwanted baseline shifts can saturate the read amplifier during long periods.
Although the saturation can be avoided by using non-linear control loops in the input stage(s) of the preamplifier, see earlier filed but not yet published European Patent Application No. 98402802.7, having a filing date Dec. 11,1998 corresponding to U.S. (PHN 17177), the residual baseline shifts can still cause loss of data over long periods of time.
By carrying out additional (fast) baseline correction, the effects of the thermal asperities can be further suppressed. In addition, it may be required to add a detection circuit to detect thermal asperities, e.g., so as to freeze the control circuits in the read out circuit, in the event that the thermal asperities would otherwise severely distort the read out circuit.